This invention relates to a brush seal element.
Brush seals are frequently used in the turbine industry to form seals between areas of high and low pressure around, for example, rotating shafts. In a typical construction an array of bristles is mounted to extend radially into a space to seal with a rotary element and, on the low pressure side of the bristles, there is a backing plate to limit the deflection of the bristles due to the pressure difference across them. As the backing plate has no further structural function and is not movable during operation, it is frequently made of rather soft material in order to keep down cost. Further the material is often also chosen so as to match the coefficient thermal expansion of the seal housing material.
The Applicants have determined that the bristles can experience galling or fretting as they move relative to the back plate and this can lead to the progressive failure of the brush seal as layer after layer of bristles are removed. The wear particles can also be damaging within the system.
From one aspect the invention consists in a brush seal element including an array of bristles and a backing member, the array extending across the face of the backing member characterised in that at least part of the surface of the face of the backing member is formed of or covered by a material which is tribologoically matched (as hereinafter defined) with at least the material of the bristles immediately adjacent the face.
For the purposes of this specification xe2x80x9ctribologically matchedxe2x80x9d means that the bristles can move on the material without inappropriate wear. This might, alternatively, be described as the material being xe2x80x9crub tolerantxe2x80x9d. In general appropriate materials will be of similar hardness to the bristle material and be relatively lubricious vis-à-vis the bristle material. By xe2x80x9csimilar hardnessxe2x80x9d it is meant that the material has a hardness of around xe2x88x9230% to +10% of the hardness of the bristles.
When tribologically matching the materials it may prove prudent in some cases to ensure that the back plate insert becomes the sacrificial part of the pair to ensure that no bristle wear takes place. In such cases, the hardness of the material may be down to around xe2x88x9250% of the hardness of the bristles. Typically, the coating is retained at the temperature of the brush seal during operation of a machine including the seal. The operating temperature of the machine can vary (the gas at a typical brush seal location is typically in the range 100xc2x0 C. to 800xc2x0 C.), but the temperature of the brush seal itself can be considerably higher due to frictional heating. This may be easily achieved with the shim or wire insert by making this material slightly softer than the hard materials of the bristles.
The element may be a one piece annular loop, round or straight seal, for a part of a multipiece segmented round or straight seal.
The element may include a perforate or open shim of tribolgically matched material located between the bristles and the backing plate. By having the shim open or perforate, thermal expansion can be accommodated within the shim without it buckling relative to the backing plate.
Alternatively an insert may be located in the face and may stand proud thereof. The insert may be welded, brazed or adhered to the face.
In a still further approach, the backing plate may be formed at least substantially of tribologically matched material, but this approach is only appropriate where the matched material has an acceptable coefficient of expansion. This will in turn depend on the construction of the seal housing.
Where the bristles are formed of a cobalt alloy the tribologically matched material may also be a cobalt alloy. Indeed, conveniently, the tribologically matched material may be the same as the bristle material.
The tribologically matched material may be deposited in the form of a coating, in which case it may be applied by arc flame or plasma spraying, detonation gun, high velocity oxygen fuel thermal spraying or physical vapour deposition. Suitable coating materials include chrome carbides or tungsten carbides.
According to another aspect of the invention there is provided a seal assembly including an insert substantially as defined above. According to yet another aspect of the invention there is provided a machine having a rotating shaft and including a seal assembly substantially as defined above.
Although the invention has been defined above, it is understood that it includes any inventive combination of the features set out above or in the following description.